Renamed create_face_neighbors_index() to its_face_edge_ids().

Renamed its_create_neighbors_index() / its_create_neighbors_index_par() to its_face_neighbors() / its_face_neighbors_par().
New variant of its_face_edge_ids() to create edge IDs from face neighbors.
Fixed some incorrect use of _NDEBUG, it should be NDEBUG.
PrintObject::slice_support_volumes() returns newly Polygons, which are cheaper than ExPolygons.
Updated SeamPlacer and SupportMaterial to use regions defined as Polygons, not ExPolygons.
TriangleSelector::get_facets_strict() returning a patch with T-joints retriangulated.
New slice_mesh_slabs() - slicing projections of a triangle patch into top / bottom layers of slices, for MMU top / bottom segmentation.
TriangleMeshSlicer - use 64 mutexes instead of one when scattering sliced triangles into layers. This makes a big difference on modern many core desktop computers.
When applying MM segmented regions to input regions, the split regions are now re-merged with 10x higher positive offset epsilon to avoid creating gaps.
When testing for existence of paint-on supports or seam, use a more efficient has_facets() test, which does not deserialize into the expensive TriangleSelector tree structure.
GLIndexedVertexArray newly uses Eigen::AlignedBox<float, 3> for efficiency instead of our double based BoundingBoxf3.
Improved MMU painting refresh speed by optimizing generation of the vertex buffers.
Refactored MMU segmentation - projection of painted surfaces from top / bottom.
	1) Parallelized.
	2) Using the new slice_mesh_slabs() instead of projecting one triangle by the other and merging them with Clipper.

src/libslic3r/TriangleMesh.hpp

@@ -116,13 +116,14 @@ private:
 // Map from a face edge to a unique edge identifier or -1 if no neighbor exists.
 // Two neighbor faces share a unique edge identifier even if they are flipped.
 // Used for chaining slice lines into polygons.
-std::vector<Vec3i> create_face_neighbors_index(const indexed_triangle_set &its);
-std::vector<Vec3i> create_face_neighbors_index(const indexed_triangle_set &its, std::function<void()> throw_on_cancel_callback);
+std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its);
+std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, std::function<void()> throw_on_cancel_callback);
+// Having the face neighbors available, assign unique edge IDs to face edges for chaining of polygons over slices.
+std::vector<Vec3i> its_face_edge_ids(const indexed_triangle_set &its, std::vector<Vec3i> &face_neighbors, bool assign_unbound_edges = false, int *num_edges = nullptr);
 
 // Create index that gives neighbor faces for each face. Ignores face orientations.
-// TODO: naming...
-std::vector<Vec3i> its_create_neighbors_index(const indexed_triangle_set &its);
-std::vector<Vec3i> its_create_neighbors_index_par(const indexed_triangle_set &its);
+std::vector<Vec3i> its_face_neighbors(const indexed_triangle_set &its);
+std::vector<Vec3i> its_face_neighbors_par(const indexed_triangle_set &its);
 
 // After applying a transformation with negative determinant, flip the faces to keep the transformed mesh volume positive.
 void its_flip_triangles(indexed_triangle_set &its);
@@ -153,6 +154,28 @@ void its_collect_mesh_projection_points_above(const indexed_triangle_set &its, c
 Polygon its_convex_hull_2d_above(const indexed_triangle_set &its, const Matrix3f &m, const float z);
 Polygon its_convex_hull_2d_above(const indexed_triangle_set &its, const Transform3f &t, const float z);
 
+// Index of a vertex inside triangle_indices.
+inline int its_triangle_vertex_index(const stl_triangle_vertex_indices &triangle_indices, int vertex_idx)
+{
+    return vertex_idx == triangle_indices[0] ? 0 :
+           vertex_idx == triangle_indices[1] ? 1 :
+           vertex_idx == triangle_indices[2] ? 2 : -1;
+}
+
+inline Vec2i its_triangle_edge(const stl_triangle_vertex_indices &triangle_indices, int edge_idx)
+{
+    int next_edge_idx = (edge_idx == 2) ? 0 : edge_idx + 1;
+    return { triangle_indices[edge_idx], triangle_indices[next_edge_idx] };
+}
+
+// Index of an edge inside triangle.
+inline int its_triangle_edge_index(const stl_triangle_vertex_indices &triangle_indices, const Vec2i &triangle_edge)
+{
+    return triangle_edge(0) == triangle_indices[0] && triangle_edge(1) == triangle_indices[1] ? 0 :
+           triangle_edge(0) == triangle_indices[1] && triangle_edge(1) == triangle_indices[2] ? 1 :
+           triangle_edge(0) == triangle_indices[2] && triangle_edge(1) == triangle_indices[0] ? 2 : -1;
+}
+
 using its_triangle = std::array<stl_vertex, 3>;
 
 inline its_triangle its_triangle_vertices(const indexed_triangle_set &its,